Starter device for a discharge lamp

ABSTRACT

A starter device for a discharge lamp instantaneously lighting a preheating type fluorescent discharge tube where in parallel to the said fluorescent discharge tube is connected a reverse blocking diode thyristor, the breakover voltage being lower than the maximum value of the rated voltage of the power source but higher than the discharge voltage of the said fluorescent discharge tube while the blocking voltage being sufficiently larger than the said breakover voltage.

United States Patent 1191 Takei et al.

1 51 Feb. 13, 1973 541 STARTER DEVICE FOR A DISCHARGE 2,871,409 1 1959Aldrich 8! al 315/106 x LAMP 3,235,769 2/1966 Wattenbach ..3l5/233 X3,383,558 5/1968 Waymouth.... ..3l5/242 [751 In e o Takei, g -k Osaka;3,407,334 10/1968 Attewell ..315/273 Yasutaka Kawai, Higashiosaka-shi;3,476,976 11/1969 Morita et a1. ..315/105 X Takeshi Matsushima,Nara-shi, all of Japan FOREIGN PATENTS OR APPLICATIONS A signee;Matsushita Electric Industrial Co. l78,4l8 3/1966 ..3l5/l00 Ltd" Osaka,Japan 1,072,717 6/1967 Great Britain ..315/100 [22] Filed: May 1969Primary ExaminerRoy Lake 21 APPL 21 11 AttameyStevens, Davis, Miller &Mosher [30] Foreign Application Priority Data [57] ABSTRACT May 21, 1968Japan ..43/35240 May 17, 1968 May 21, 1968 A starter device for adischarge lamp instantaneously May 24, 1968 Japan ..43/35853 lighting apreheating type fluorescent discharge tube May 24, 1968 Japan ..43/43740where in parallel to the said fluorescent discharge tube is connected areverse blocking diode thyristor, the "MS/1235382612 breakover voltagebeing lower than the maximum o I s n n u n u u u u u v u u u e 1 6 p 6 1l 6 u u u u u u u e I a u v [58] 6;fig :88 2 65 2 higher than thedischarge voltage of the said 233 235 307/302 fluorescent discharge tubewhile the blocking voltage being sufficiently larger than the saidbreakover voltage.

[56] References Cited UNITED STATES PATENTS 9 Claims, 20 Drawing Figures2,842,715 7/1958 Charbonneaux ..3l5/l84 c J b LQJ =5 PATENTEDFEBI 31815SHEET u BF 4 3.? 16.751

STARTER DEVICE FOR A DISCHARGE LAMP This invention generally relates toa starter device for a discharge tube, and more particularly to astarter device for lighting a preheating fluorescent discharge tubewhere in parallel to the fluorescent discharge tube a reverse blockingdiode thyristor having a blocking voltage much larger than the breakovervoltage is connected.

A conventional starter device as shown in FIGS. 1 and 2 consists of amanual switch 2 or a glow switch 3 connected in parallel to thefluorescent discharge tube, an a.c. source 4, a switch and a chokeballast 6. A manual switch is inconvenient because it requires much timebefore starting while a glow switch has the problem of a shorterlifetime although the starting time is fast.

Therefore, this invention is aimed at removing the above-mentioneddefects of the prior art and provides a novel starter device for adischarge tube.

The object of this invention is to provide a starter device for rapidlystarting the fluorescent discharge tube where in parallel to thefluorescent discharge tube is connected a reverse blocking diodethyristor, the breakover voltage being lower than the maximum value ofthe rated voltage of the power source but higher than the operatingvoltage of the fluorescent discharge tube while the blocking voltagebeing sufficiently larger than the breakover voltage.

' This invention has the following advantages:

1. The life of the starter is semipermanent because it is made of asemiconductor element.

2. The element has the advantage of being more economical.

3. Rapid starting is possible with the current limiting ballast.

4. The circuit construction is simple and minuature, and is thereforevery favorable for production.

5. In comparison with a conventional device with a current limitingballast for rapid starting, the inventive device is small in size andlight in weight because a usual current limiting ballast can beemployed.

6. Due to the electrical switching action, damage caused to thefluorescent discharge tube is smaller compared to a glow switch or amanual starter.

The invention will be explained hereunder with reference to thedrawings, in which:

FIGS. 1 and 2 are the electric circuit diagrams of prior art glow switchand manual starter devices for a discharge tube;

FIG. 3 shows the electric circuit diagram of a starter device for adischarge tube according to one embodiment of this invention;

FIG. 4 shows the voltage-current characteristic of a reverse blockingdiode thyristor used in the inventive device;

FIG. 5 shows the waveforms across the terminals 0 and b of the deviceshown in FIG. 3 during preheating and operating;

FIG. 6 shows the waveform of current flowing through the thysistor inthe above device during preheating;

FIGS. 7 and 8 show the voltage current characteristics of a symmetricaldiode thyristor and a diode respectively, which are used for obtainingthe reverse blocking diode thyristor having the voltage-currentcharacteristics shown in FIG. 4;

FIG. 9 shows an electric circuit diagram of a starter device for adischarge tube according to another embodiment of this invention;

FIG. 10 shows the waveform of the voltage across both terminals of thedischarge tube of FIG. 9 during the preheating and operating period ofthe tube;

FIG. 11 shows the waveform of the current flowing through the thyristorin the above device during preheating;

FIG. 12 shows an electric circuit diagram of a starter device for adischarge tube according to anotherembodiment of this'invention;

FIG. 13 shows the waveform of the voltage across both terminals of theabove discharge tube during preheating and operating;

FIG. 14 shows the waveform of the current flowing through the thyristorin the above device during preheating;

FIG. 15 shows an electric circuit diagram of a starter device for adischarge tube according to another embodiment of this invention;

FIG. 16 shows the waveform of the voltage across both terminals of theabove discharge tube during preheating and operating;

FIG. 17 shows the waveform of the current flowing through the thyristorin the above device during preheating;

FIG. 18 shows the voltage current characteristic of a reverse blockingdiode thyristor used for obtaining a starter; and

FIGS. 19 and 20 show the sectional and front view of a reverse blockingdiode thyristor according to one embodiment of the starter device ofthis invention.

An embodiment of this invention will be explained with reference toFIGS. 3 to 8 hereinafter. In FIG. 3, 7 is an a.c. power source, 8 is afluorescent discharge tube both terminals of which are connected to thea.c.

power-source through a switch 9 and a current limiting ballast 10, 11 isa reverse blocking diode thyristor connected in parallel to thedischarge tube 8, and 12 is a noise preventing capacitor connected inparallel to the a.c. power source 7. The reverse blocking diodethyristor 11 has such a switching characteristic as shown in FIG. 4.Namely, at a given forward voltage (breakover voltage V) the reverseblocking diode thyristor changes from the cut-off state to theconducting state. The breakover voltage V satisfies the condition:maximum value of the rated output voltage of the power source breakovervoltage V the peak value of the operating voltage of the discharge tube8, while the blocking voltage V satisfies the.condition; V the maximumvalue of the rated output voltage of the power source.

The reverse blocking diode thyristor 11 having the voltage-currentcharacteristics shown in FIG. 4 can be obtained by connecting in seriesa symmetrical diode thyristor having the voltage-current characteristicsshown in FIG. 7 and a diode having the characteristics shown in FIG. 8,the former having the characteristic: maximum value of the rated outputvoltage of the power source 7 breakover voltage V the peak value of theoperating voltage of the discharge tube 8, while the latter having thecharacteristic: blocking voltage V maximum value of the rated outputvoltage of the power source 7.

The operation of the reverse blocking diode thyristor will be explainedreferring to FIGS. 5 and 6. In the circuit in FIG. 3, when the switch 9is on, the rated output voltage of the power source 7 is higher than thebreakover voltage V 50 is applied across the terminals a and b of thereverse blocking diode thyristor 11. The breakdown of the reverseblocking diode thyristor 11 makes the terminal a b conducting. Thefilament of the fluorescent discharge tube 8 reaches the preheatingstate. When the reverse blocking diode thyristor 11 changes from theconducting state to the cut-off state in each positive cycle of thepower source, a high voltage is applied across both filaments of thefluorescent discharge tube8 in the form of a kick voltage. Each negativecycle is blocked. By the half wave rectification a dc. current issuperimposed on the circuit. The current limiting ballast 10 causes themagnetic saturation and a sufficient preheating current flows throughthe electrode. in each negative half cycle the reverse blocking diodethyristor does not breakover so that the power source voltage is appliedacross the discharge tube 8 and effects starting. After several cyclesof the filament preheating, the kick voltage, and the source voltageapplication, the discharge tube 8 begins to discharge and ignite. Theturn-off of the conducting reverse blocking diode thyristor 11 isdetermined by the current flowing through the reverse blocking diodethyristor 11. in the circuit,'the current lags behind the voltage due tothe inductance of the current limiting ballast 10. So, the conduction ofthe thyristor is maintained during the initial part of the negative halfcycle. Once the discharge begins, the voltage across terminals or and bof the reverse blocking. diode thyristor 11 becomes equal to theoperating voltage of the fluorescent discharge tube 8. The reverseblocking diode thyristor reaches cut-off and the fluorescent dischargetube 8 remains in the on state. The noise preventing capacitor 12,usually connected in parallel to the starter, often gives a highervoltage than the breakover voltage V across the thyristor, whereby thethyristor is reignited. This is due to the charging and discharging ofthe capacitor during the lighting of the discharge tube 8, occurringwhen the assembly using this starter is used under such bad conditionsthat the temperature is decreased abnormally or the source voltage israised abnormally. Because of this fact in this invention a noisepreventing capacitor 12 of the order of 0.2 u is inserted between thepoints 0 and d, i.e., between the input of the current limiting ballast10 and the electrode input of the fluorescent discharge tube 8 whereby asimilar effect is obtained.

Explanation of modified circuit diagrams with various auxiliary circuitsadded to the above basic circuit, where the reverse blocking diodethyristor is connected in parallel to the fluorescent discharge tube asa starter, will be made with reference to FIGS. 9 to 17. The reverseblocking diode thyristor used hereinafter has the characteristic asexplained above, and may be a combination of a thyristor and a diode ora single element, included within the general term. reverse blockingdiode thyristor.

F K]. 9 shows a combination of a current limiting ballast and tworeverse blocking diode thyristors. One end of the current limitingballast 13 is connected to one end of the power source 14 while theother end thereof is connected to one end of the filament 16 of thefluorescent discharge tube 15 and to one end of reverse blocking diodethyristor 17. The other end of the filament 16 is connected to one endof the other filament 19 through the reverse blocking diode thyristor18. One end of the filament 19 is connected to the other end of thepower source 14 through a switch 20. The other end of the reverseblocking diode thyristor 17 is connected to the other end of thefilament 19. A noise preventing capacitor 21 is inserted between theinput side of the current limiting ballast B and the output side of theswitch 20. The reverse blocking diode thyristors l7 and 18 are connectedoppositely to each other, their characteristics being the same aspreviously stated.

The operation of the circuit of FIG. 9 is as follows. When the switch 20is closed, the power source voltage is applied to the reverse blockingdiode thyristors 17 and 18. If the voltage on the side of currentlimiting ballast is positive, the reverse blocking diode thyristor 18reaches the breakover voltage V and changes from the cut-off to theconducting state. The reverse blocking diode thyristor 17 maintains theblocking state due to its opposite polarity. Current flows in thedirection of the current limiting ballast l3 the filament 16 the reverseblocking diode thyristor 18. The reverse blocking diode thyristor 17becomescut-off. No current flows through the filament 19, the preheatingcurrent flowing only through the filament 16. Next, when the polarity ofthe current limiting ballast 13 becomes negative, the reverse blockingdiode thyristor 18 becomes off while the reverse blocking diodethyristor 17 becomes on. The current flows in the direction of thefilament l9 the reverse blocking diodethyristor 17 the current limitingballast 13. It is designed such that the time when the reverse blockingdiode thyristor 17 changes from the on to off state does not coincidewith the time when the reverse blocking diode thyristor 18 becomes on sothat a pulse voltage is applied between the filaments 16 and 19 as shownin FIG. 10. The current waveform becomes as shown in H6. 11. This timelag also exists between the time when the reverse blocking diodethyristor 18 changes from the on to off state and the time when thereverse blocking diode thyristor 17 becomes on. Thus, the current flowsthrough the filaments 16 and 19 alternately. Pulse voltages are appliedtherebetween, whereby the fluorescent discharge tube 15 starts ignition.Once the discharge tube 15 is started, the reverse blocking diodethyristors 17 and 18 becomes cut-off due to the condition: the maximumvalue of the rated output voltage of the power source 14 breakovervoltage V of the reverse blocking diode thyristors 17 and 18 operatingvoltage of the discharge tube 15. So, the discharge is maintained. Thestart of the fluorescent discharge tube 15 is thus attained by theelectrical switching action of the reverse blocking diode thyristors 17and 18 so that the time required for the start is extremely short.

In FIG. 12, in order to increase the voltage applied to the dischargetube at starting a series connection of'an auxiliary capacitor forstarting and a symmetrical diode thyristor is connected in parallel tothe fluorescent discharge tube. with only a capacitor the charge anddischarge of the capacitor after operating of the fluorescent dischargetube makes the peak value of the voltage waveform as high as thebreakover voltage V of the reverse blocking diode thyristors andoccasionally causes reignition. To prevent this the symmetrical diodethyristor is connected in series to form an auxiliary circuit forstarting. The breakover voltage V M of the symmetrical diode thyristoris equal to or larger than that of the reverse blocking diode thyristor.A further necessary condition is; the maximum value of the rated outputvoltage of power source breakover voltage V 80 of the symmetrical diodethyristor operating voltage of the discharge tube. By this arrangementthe auxiliary capacitor for starting becomes electrically cut-off afterthe start of the tube operation. Thus, the circuit consisting ofcapacitor and the symmetrical diode thyristor operates only at thestarting time. In FIG. 12, one end of the current limiting ballast 22 isconnected to one end of the power source 23 and the other end thereof isconnected to one end of the filament 25 of the fluorescent dischargetube 24 and to a capacitor 26 of the auxiliary circuit. The capacitor 26is connected in series to the symmetrical diode thyristor 27, and thiscombination is connected between the filaments 25 and 28 of thedischarge tube 24. The reverse blocking diode thyristor 29 is alsoconnected between the filaments 25 and 28 of the discharge tube 24. Theother end of the filament 28 is connected to the other end of the powersource through a switch 30. The noise preventing capacitor 31 isconnected between the input side of the current limiting ballast 22 andthe output side of the switch 30.

The operation of the circuit in FIG. 12 is as follows. When the forwardvoltage applied to the reverse blocking diode thyristor 29 reaches thebreakover voltage V of the revere blocking diode thyristor 29, thecurrent flows in the direction of current limiting ballast 22 filament25 reverse blocking diode thyristor 29 filament 28. In this case thecapacitor 26 of the auxiliary circuit is charged and at the same timethe symmetrical diode thyristor 27 reaches the breakover voltage V Theimpedance of the auxiliary circuit consists only of the impedance of thecapacitor 26, and is much larger than the resistances of the filaments25 and 28. Therefore, the current flowing through this circuit is small.When a reverse voltage is applied, no current flows through thefilaments 25 and 28. The charging voltage of the capacitor 26 issuperposed on the voltage of the power source 23, and hence the voltageapplied to the fluorescent discharge tube 24 becomes higher than thepower source voltage. The above action is repeated in accordance withthe power source cycles whereby the preheating is effected. The highvoltage together with the preheaing makes the discharge tube 24 start.Once after starting the discharge tube the reverse blocking diodethyristor and the symmetrical diode thyristor become both cut-off astheir breakover voltages V are higher than the operating voltage of thedischarge tube. Thus, the discharge is maintained. FIGS. 13 and 14 showthe waveforms of the voltage and the current across the terminals e andf during preheating and operating. The characteristic of the symmetricaldiode thyristor 27 is shown in FIG. 7.

FIG. shows a starter circuit for a high output fluorescent dischargetube of the rapid starting type which uses a reverse blocking diodethyristor and an auxiliary capacitor for starting in the high outputcircuit. 32 is a power source, 33 is a current limiting ballast havingfour coils 34, 35, 36 and 37, 38 is a switch, 39 is a fluorescentdischarge tube having filaments 40 and 41 on both ends, 42 is acapacitor, and 43 is a reverse blocking diode thyristor. In this circuitcomposition, one filament 40 is connected to the coils 34 and 35 of thecurrent limiting ballast 33. The other ends of these coils are bothconnected to the power source 32. The filament 41 is connected to thecoils 36 and 37 of the current limiting ballast 33. The other ends ofthese coils are both connected to the power source 32 through the switch38. The capacitor 42 is inserted between the connection point of thefilament 40 and the coil 34 and the connection point of the filament 41and the coil 37. The reverse blocking diode thyristor 43 is insertedbetween the connection point of the filament 40 and the coil 35 and theconnection point of the filament 41 and the coil 36. The operation ofthis circuit is as follows. When the switch 38 is closed and a positivevoltage is applied to the reverse blocking diode thyristor 43 by thepower source 32, the composite currents flow through the coil 34 and thefilament 40 and through the coil 35 via the reverse blocking diodethyristor 43 and the coil 36 and through the filament 41 and the coil37. The capacitor 42 due to its high impedance allows no substantialcurrent to pass. Thus, the preheating current flows in the direction ofcoil 34 of the current limiting ballast 33 filament 40 reverse blockingdiode thyristor 43 filament 41 coil 37. During the next negative voltagecycle the reverse blocking diode thyristor 43 becomes cut-off so thatthe voltage is applied only to the filament 40 and 41. By charging, thevoltage of the capacitor 42 reaches a high voltage across the terminal.The voltage is applied at the filament 40 and 41 as a starting voltageand the discharge tube 39 is started. The operation is done inaccordance with the frequency of the power source 32, and hence thefluorescent discharge tube 39 is rapidly started. Once the dischargetube is started, the voltage across the reverse blocking diode thyristor43 becomes equal to the operating voltage of the tube and makes thereverse blocking diode thyristor 43 cut-off. So, the discharge of thefluorescent discharge tube 39 is maintained. FIGS. 16 and 17 show thewaveforms of the voltage and the current between the terminals g and hof the discharge tube during preheating and operating.

The reverse blocking diode thyristor used in this invention has thesecharacteristics: i.e., the breakover voltage for the forward direction Vthe blocking voltage for the backward direction V The mass production ofsuch reverse blocking diode thyristors is very difficult from theviewpoints of manufacturing technique and economy. The use of a siliconcontrolled rectifier element instead of the reverse blocking diodethyristor is complicated because of the necessity of the accessarycircuit, and furthermore it is not so economical.

Hereinafter, explanation will be made of a method for obtaining theabove-mentioned reverse blocking diode thyristor with reference to FIGS.7, 8 and 18. A symmetrical diode thyristor whose characteristic is asshown in FIG. 7 and a publicly known diode whose characteristic is asshown FIG. 8 are used. The symmetrical diode thyristor changes from thecut-off to the conducting state when the terminal voltage exceeds acertain value (the breakover voltage V The characteristic is inprinciple symmetric with respect to the voltage axis. The blockingvoltage V R of the diode is assumed to be larger than the breakovervoltage V 50 of the symmetrical diode thyristor. Connected in serieswith the above two semiconductors and sealed together in a plastic ormetal'case, a reverse blocking diode thyristor whose characteristic isshown in FIG. 4 is situated. The characteristic of the reverse blockingdiode thyristor in the forward direction is equal to that of thesymmetrical diode thyristor while that in the backward direction isequal to that of the diode. The blocking voltage V R is sufficientlylarger than the breakover voltage V instead of the symmetrical diodethyristor a conventional reverse blocking diode thyristor whosecharacteristic is as shown in FIG. 18, i.e., the breakover voltage V inthe forward direction is nearly equal to the blocking voltage V in thereverse direction, may be used for the same efi'ect by connecting it inthe same direction in series with the diode of FIG. 8 and sealing themin a plastic or metal case.

FIGS. 19 and 20 show reverse blocking diode thyristors thus obtained.Numeral 44 is a plastic or metal case, 45 is the diode, 46 is thesymmetric diode thyristor, and 47 is the reverse blocking diodethyristor whose breakover voltage V is nearly equal to the blockingvoltage V The two semiconductors are individually sealed in a metal orplastic case and the series connection from them is also sealed in asimilar case. Alternatively, lead wires are titted to the unsealedsemiconductors to connect them in series and this combination is sealedin a metal or plastic case.

Considering that the reverse blocking diode thyristors appearing inFIGS. 3 to 17 are obtained by sealing a series connection of either asymmetric diode thyristor or a reverse blocking diode thyristor having abreakover voltage V nearly equal to the blocking voltage V with a diodehaving a blocking voltage V,,'

fluorescent discharge tube, comprising: an AC power source; afluorescent discharge tube connected across said AC source and havingfirst and second opposed filaments; reverse blocking diode thyristormeans connected between a first terminal of each of said filaments ofsaid discharge tube, said thyristor means having a blocking voltagewhich is higher than the maximum rated output voltage of said AC sourceand a breakover voltage which is lower than said maximum rated voltageand higher than the peak value of the operating voltage of saiddischarge tube; switch means connected between one side of said ACsource and a second terminal of said first filament; current limitingmeans connected between the other side of said AC source and a secondterminal of said second filament;

and capacitor means connected across said AC source. 2. Apparatusaccording to claim 1, further comprising: a symmetrical diode thyristorand a starting capacitor connected in series between said secondterminals of said first and second filaments, said symmetrical diodethyristor having a breakover voltage lower than said maximum ratedoutput voltage and higher than said peak operating voltage of said tube.

3. Apparatus according to claim 2, wherein said thyristor meanscomprises a single reverse blocking diode thyristor.

4. Apparatus according to claim 2, wherein said thyristor meanscomprises a series connection of a symmetrical diode thyristor and adiode, said symmetrical diode thyristor having said breakover voltageand said diode having said blocking voltage.

5. Apparatus according to claim 2, wherein said thyristor meanscomprises a series connection of a reverse blocking diode thyristor anda diode, said reverse blocking diode thyristor having said breakovervoltage and a blocking voltage nearly equal to said breakover voltage,and said diode having a blocking voltage higher than the power sourcevoltage.

6. Apparatus, including a circuit for starting a fluorescent dischargetube, comprising: an AC source; a fluorescent discharge tube havingopposed filaments and connected across said AC source; reverse blockingdiode thyristor means connected directly across said filaments andacross said AC source, said thyristor means having a blocking voltagewhich is higher than the maximum rated output voltage of said AC sourceand a breakover voltage which is lower than said maximum rated voltageand higher than the peak value of the operating voltage of saiddischarge tube; switch means connected between one side of said ACsource and a terminal of one of said filaments; current limiting meansconnected between the other side of said AC source and a terminal of theother filament; and capacitor means connected across said AC source.

7. Apparatus according to claim 6, wherein said thyristor meanscomprises a single reverse blocking diode thyristor.

8. Apparatus according to claim 6, wherein said thyristor meanscomprises a series connection of a sym metrical diode thyristor and adiode, said symmetrical diode thyristor having the breakover voltagelower than said maximum rated output voltage of said AC source andhigher than said peak value of the operating voltage of the dischargetube, and said diode has a blocking voltage higher than said maximumrated output voltage.

9. Apparatus according to claim 6 wherein said thyristor means comprisesa series connection of a reverse blocking diode thyristor and a diode,said diode thyristor having the breakover voltage lower than saidmaximum rated voltage of said AC source and higher than said peak valueof said operating voltage and hav ing a blocking voltage nearly equal tosaid breakover voltage, said diode having a blocking voltage higher thanthe power source voltage.

1. Apparatus, including a circuit for starting a fluorescent dischargetube, comprising: an AC power source; a fluorescent discharge tubeconnected across said AC source and having first and second opposedfilaments; reverse blocking diode thyristor means connected between afirst terminal of each of said filaments of said discharge tube, saidthyristor means having a blocking voltage which is higher than themaximum rated output voltage of said AC source and a breakover voltagewhich is lower than said maximum rated voltage and higher than the peakvalue of the operating voltage of said discharge tube; switch meansconnected between one side of said AC source and a second terminal ofsaid first filament; current limiting means connected between the otherside of said AC source and a second terminal of said second filament;and capacitor means connected across said AC source.
 1. Apparatus,including a circuit for starting a fluorescent discharge tube,comprising: an AC power source; a fluorescent discharge tube connectedacross said AC source and having first and second opposed filaments;reverse blocking diode thyristor means connected between a firstterminal of each of said filaments of said discharge tube, saidthyristor means having a blocking voltage which is higher than themaximum rated output voltage of said AC source and a breakover voltagewhich is lower than said maximum rated voltage and higher than the peakvalue of the operating voltage of said discharge tube; switch meansconnected between one side of said AC source and a second terminal ofsaid first filament; current limiting means connected between the otherside of said AC source and a second terminal of said second filament;and capacitor means connected across said AC source.
 2. Apparatusaccording to claim 1, further comprising: a symmetrical diode thyristorand a starting capacitor connected in series between said secondterminals of said first and second filaments, said symmetrical diodethyristor having a breakover voltage lower than said maximum ratedoutput voltage and higher than said peak operating voltage of said tube.3. Apparatus according to claim 2, wherein said thyristor meanscomprises a single reverse blocking diode thyristor.
 4. Apparatusaccording to claim 2, wherein said thyristor means comprises a seriesconnection of a symmetrical diode thyristor and a diode, saidsymmetrical diode thyristor having said breakover voltage and said diodehaving said blocking voltage.
 5. Apparatus according to claim 2, whereinsaid thyristor means comprises a series connection of a reverse blockingdiode thyristor and a diode, said reverse blocking diode thyristorhaving said breakover voltage and a blocking voltage nearly equal tosaid breakover voltage, and said diode having a blocking voltage higherthan the power source voltage.
 6. Apparatus, including a circuit forstarting a fluorescent discharge tube, comprising: an AC source; afluorescent discharge tube having opposed filaments and connected acrosssaid AC source; reverse blocking diode thyristor means connecteddirectly across said filaments and across said AC source, said thyristormeans having a blocking voltage which is higher than the maximum ratedoutput voltage of said AC source and a breakover voltage which is lowerthan said maximum rated voltage and higher than the peak value of theoperating voltage of said discharge tube; switch means connected betweenone side of said AC source and a terminal of one of said filaments;current limiting means connected between the other side of said ACsource and a terminal of the other filament; and capacitor meansconnected across said AC source.
 7. Apparatus according to claim 6,wherein said thyristor means comprises a single reverse blocking diodethyristor.
 8. Apparatus according to claim 6, wherein said thyristormeans comprises a series connection of a symmetrical diode thyristor anda diode, said symmetrical diode thyristor having the breakover voltagelower than said maximum rated output voltage of said AC source andhigher than said peak value of the operating voltage of the dischargetube, and said diode has a blocking voltage higher than said maximumrated output voltage.
 9. Apparatus according to claim 6 wherein saidthyristor means comprises a series connection of a reverse blockingdiode thyristor and a diode, said diode thyristor having the breakovervoltage lower than said maximum rated voltage of said AC source andhigher than said peak value of said operating voltage and having ablocking voltage nearly equal to said breakover voltage, said diodehaving a blocking voltage higher than the power source voltage.